An exemplary application for the techniques disclosed herein is that of ink-jet printing. By way of example, thermal inkjet printers operate by expelling a small volume of ink through a plurality of small nozzles or orifices in a surface held in proximity to a medium upon which marks or printing is to be placed. These nozzles are arranged in a fashion in the surface such that the expulsion of a droplet of ink from a determined number of nozzles relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the substrate or the medium and another expulsion of ink droplets continues the production of more pixels of the desired character or image. Inks of selected colors may be coupled to individual arrangements of nozzles so that selected firing of the orifices can produce a multicolored image by the inkjet printer.
Expulsion of the ink droplet in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature which exceeds the boiling point of the ink solvent and creates a vapor phase bubble of ink. Rapid heating of the ink can be achieved by passing a square pulse of electric current through a resistor, typically for 0.5 to 5 microseconds. Each nozzle is coupled to a small unique ink firing chamber filled with ink and having the individually addressable heating element resistor thermally coupled to the ink. As the bubble nucleates and expands, it displaces a volume of ink which is forced out of the nozzle and deposited on the medium. The bubble then collapses and the displaced volume of ink is replenished from a larger ink reservoir by way of ink feed channels.
After the deactivation of the heater resistor and the expulsion of ink from the firing chamber, ink flows back into the firing chamber to fill the volume vacated by the ink which was expelled. It is desirable to have the ink refill the chamber as quickly as possible, thereby enabling very rapid firing of the nozzles of the printhead. The ink flow into the chamber is through an entrance channel.